The invention relates to a power-split transmission for driving a first and a second axle of a motor vehicle, in the case of which transmission are located a first and a second output shaft and a controllable friction clutch, the torque transmitted by the friction clutch being capable of being varied by the action of pressure upon a pressure plate.
Power-split transmissions are used in various configurations. They can be installed with or without a central differential, with a differential lock or a connectable drive of the second driven axle, with or without a reduction stage, and, finally, also longitudinally or transversely. The use of a friction clutch also makes it possible, in addition to smooth engagement, to have continuous control of the torque transmitted by the latter.
For engagement, the clutch is acted upon, this requiring considerable actuating forces. These forces are normally applied hydraulically. However, hydraulic actuation is too slow to release the clutch in the short shift time necessary for electronic slip limitation systems (ESB, ABS, etc.). Moreover, passenger cars do not possess a hydraulic system. Nonhydraulic actuation, for example by electrical means and mechanical transmission, have hitherto come up against the problem of the high actuating forces which have had to be introduced into the case via bearings.
Also, where electrical actuation is concerned, there is always the question of operating reliability in the event of failure of the electrical means. The system must be intrinsically safe, that is to say the more reliable switching state must always be assumed if the electrics fail. In power-split transmissions with a reduction stage (off-highway gear stage), in addition to the friction clutch, there is, furthermore, the problem also of activating the latter in coordination with the differential lock or axle-drive connection.
The object of the invention is, therefore, to improve power-split transmissions to the effect that they can be controlled quickly, in order to achieve the short shift times necessary for RSB, and to the effect that no actuating forces have to be introduced into the case, said power-split transmissions being intrinsically safe in functional terms, at a minimal outlay in terms of construction.
The foregoing object is achieved, according to the invention wherein
a) for the action of pressure upon the pressure plate, there are provided two rotationally uncoupled ring parts which are coaxial to the friction clutch and at least one of which possesses ramps rising in the circumferential direction,
b) the two ring parts are supported in the axial direction and each possess a ramp lever, the free end of which cooperates with the control unit which possesses electrical drive means and which moves the ramp levers in opposite directions,
c) the control unit having a rotatable control disk with two slots, and the drive means being an electric geared motor.
The rings rotationally uncoupled from one another and from the shaft carrying them are rotatable relative to one another by means of their ramp levers, and, because of the oppositely directed movement of these, no actuating forces have to be diverted to the case. The rising ramps in the ring/rings require only very low relative rotation for high actuating forces and, with rolling bodies being interposed, are frictionless, so that very short shift times are achieved in cooperation with the electrical actuation. The electric geared motor has a very small build, if the motor rotational speed and transmission ratio are appropriately selected, and can be controlled quickly by known means. The construction of the two slots on a rotatable control disk ensures the oppositely directed movement under all circumstances.
At the same time, a particularly space-saving form of construction is obtained when the axis of rotation of the control disk is oriented approximately in the direction of movement of the free ends of the two ramp levers. Moreover, it is thus possible also to actuate a reduction stage, present if appropriate, from the same geared motor and to control said reduction stage in coordination and in an intrinsically safe manner at the lowest possible outlay.
In an embodiment of the power-split transmission with a reduction stage which is capable of being shifted by axial displacement of one of its elements, there is provision according to the invention for
a) the control unit to consist of a carrier shaft driven by the geared motor, of a changeover sleeve connected fixedly in terms of rotation to said carrier shaft, but displaceable, of the control disk mounted rotatably on the carrier shaft and of a control pinion, likewise mounted rotatably on said control disk, for the changeover of the reduction stage,
b) the changeover sleeve being displaceable by means of an electromagnet and possessing first and second coupling teeth, by means of which said changeover sleeve selectively makes the drive connection to the control disk or to the control pinion.
By means of this refinement, both the friction clutch and the reduction stage can selectively be activated in an intrinsically safe manner by means of a single control unit and a single motor. Moreover, costs and construction space are saved. Furthermore, it is ensured that the two are not switched simultaneously and that the friction clutch is released in the event of failure of the control unit.
In an advantageous development, the electromagnet is a three-position magnet which displaces the changeover sleeve by means of a shift fork, and the changeover sleeve possesses shift teeth with a push-away meshing angle. As a result, in the event of failure of the electromagnet, the clutch is opened automatically, with the effect of intrinsically safe actuation.
In a particularly advantageous design of the power-split transmission with a differential lockable by means of the friction clutch and with a reduction stage, there being in the differential case a first member driven by an input shaft and in each case a driven member for driving a first and a second output shaft, the differential case having mounted in it, in addition to the balancing wheels, planet wheels of the reduction stage which mesh, on the one hand, with a driving sun wheel and, on the other hand, with a ring gear, the ring gear being displaceable in the axial direction. The differential and the reduction stage are thereby combined in structural terms, with the result that the common control of the two is also simplified.
In an advantageous development of the control of the friction clutch, a detaining disk with at least one detaining recess is connected fixedly in each case to the hub of the control disk and to the control pinion, said detaining recess cooperating with a detaining member coupled to the changeover sleeve, which detaining member possesses a detaining tooth on each side, so that the detaining tooth meshes into the detaining recess of the detaining disk when the control disk is uncoupled, and so that the detaining tooth meshes into the detaining recess of the detaining disk when the control pinion is uncoupled. This gives rise, without any external action, to the inherent safety that the driven element uncoupled in each case is detained and that it can be uncoupled only after the end of a shift meshing.
In a modified embodiment of the control,
a) the rotatable control disk has a first clutch half of a claw clutch and is rotatable and displaceable on a carrier shaft driven by the electric geared motor,
b) the second clutch half of the claw clutch is mounted fixedly on the carrier shaft, and
c) a forked lever mounted pivotably in the case engages on the control disk.
The control disk is therefore displaced axially for the engagement of the claw clutch. However, because of the oppositely directed movement of the two ramp levers, the displacement of the control disk does not have an effect on the functioning of said claw clutch. Thus, a particularly simple claw clutch with only a few components is provided. The forked lever can cooperate with an electromagnet and thus, when current is applied, can engage the claw clutch counter to the force of a spring and, if appropriate, also retain said claw clutch, so that, when current is not applied, the claw clutch is disengaged by the spring, the claw clutch having a deflector toothing (11).
In a developed variant, the carrier shaft possesses a radially projecting coupling bolt which cooperates with a control face of the control disk in such a way that, during rotation of the carrier shaft, the control disk is intermeshed by means of its first clutch half with the second clutch half, and the electromagnet is designed as a magnetic clamp. The engagement of the claw clutch is thus initiated automatically when the geared motor is started up, and the claw clutch is retained by the holding magnet as long as current is applied to said claw clutch. The design as a magnetic clamp has the advantage that only a very low magnetizing current is necessary.